There is known a communication standard called WUSB (Wireless Universal Serial Bus) in which communication is performed by the time division multiple access (TDMA) method (wireless USB specifications 1.0).
In the wireless communication system of the WUSB standard, the host and device form a WUSB cluster, and communication is performed by using a superframe of the time division multiple access base. A single superframe is structured by 256 media access slots (MASs) of 256 μs each, where for the first 16 MASs, only beacons are sent as part of beacon period (BP). The remaining period is reserved in each cluster as distributed reservation periods (DRPs, 500, 510, 520, 530, 540, 550, 551, 560, 561), which are communication bands. The period of a DRP is indicated by a DRP information element (DRP IE) inside a beacon sent from a host or a device.
The synchronization management of a host and a device is performed in an autonomous distributed manner, and the host has the function of performing synchronization management of a superframe. The host and device perform synchronization adjustment of the superframe in response to receiving a beacon from an external apparatus. However, there may be devices which exist as part of the wireless communication system of the WUSB standard but which do not perform superframe synchronization adjustment. Devices are divided broadly, as described below, according to the superframe synchronization adjustment management function.                A self-beaconing device (SBD) which itself performs superframe synchronization adjustment management.        A directed beaconing device (DBD) which itself does not perform superframe synchronization adjustment management.        A non beaconing device (NBD) which does not itself perform synchronization management, and, in order to reduce power consumption, does not perform sending and receiving of beacons.        
A plurality of WUSB clusters can exist in a wireless communication system of the WUSB standard. It is permissible for these clusters to overlap (i.e., apparatuses such as hosts and devices may form a plurality of clusters). To enable time division multiple access in this kind of system, the hosts and devices mutually establish synchronization in a superframe (i.e., perform synchronization and DRP reservation). A detailed description of the operations involved in establishing synchronization is given below. In addition, if a cluster contains only a host and a WUSB device that operates as an SBD, the host does not require operations related to establishing superframe synchronization adjustment in a WUSB layer, as the SBD mainly controls processes related to establishing synchronization adjustment.
FIG. 2 is a diagram showing an example of a plurality of WUSB clusters having overlapping parts. In FIG. 2, reference numeral 210 is a WUSB host which functions as a host in a WUSB cluster 200. Reference numeral 220 is a WUSB device which functions as a device. In FIG. 2, there is only one WUSB device controlled by the WUSB host, but it is permissible for there to be a plurality of WUSB devices if necessary. The communication system of FIG. 2 comprises two WUSB clusters 200 and 201, which comprise WUSB hosts 210 and 211 and a WUSB device 220. In FIG. 2, a different WUSB host 211 forms the WUSB cluster 201 and the WUSB device 220 is positioned to be within the communication range of the WUSB cluster 200 and 201.
Next, the frame structure of a media access control (MAC) layer used by the WUSB standard will be described with reference to FIG. 3. FIG. 3 is a diagram schematically showing the format of a superframe of the WUSB standard.
In the WUSB standard, the communication duration is managed in a frame unit called superframe (300, 301, 302, 303). A superframe is composed of 256 MASs 350, each of which is 256 μs. Therefore, the duration of one superframe is 65,536 μs. The 16 MASs at the start of the superframe are assigned as a BP (400 to 419). By sending a beacon to within the BP 400, the WUSB host and SBD reserves the band in the superframe as a DRP. The starting point of the superframe (i.e., the starting point of the BP 400) is called a beacon period start time (BPST) 4100. A beacon 700 comprises a beacon group (BG) parameter 701, a DRP IE (702) and other information elements (IEs) 703, and notifies the position of the MAS reserved using the DRP IE 702.
During the BP, an SBD not only sends a beacon, but also receives and analyzes beacons from other devices during other beacon slots. Consequently, the SBD consumes a great amount of power during execution of superframe synchronization adjustment management.
Here, apparatuses that are going through superframe synchronization adjustment at the MAC layer level are defined as neighbors, and apparatuses that are not going through synchronization are defined as aliens.
Next, the relationship between a MAC layer and a WUSB channel of the WUSB standard will be described with reference to FIG. 4. FIG. 4 is a schematic diagram showing mapping from a WUSB channel to MAC layer channel reservation.
In FIG. 4, each DRP (420, 430, 440, 450, 460, 470) corresponds to a communication reservation period within a WUSB cluster. In each of these DRPs, a micro-scheduled management command (MMC, 421, 423, 431, 433, 441, 443, 445, 451, 453, 461, 463, 471), which controls the input-output direction, is broadcast by a WUSB host. Here, each MMC comprises a header and a plurality of IEs. The interval between an MMC and another MMC is called a transaction group (TG). For example, the TG of relevance to an MMC 443 is a TG 444.
Next, FIG. 5 will be referenced to describe the superframe synchronization adjustment performed when the WUSB host 210 forms the WUSB cluster 200 in the case where the WUSB device 220 in FIG. 2 is an SBD. FIG. 5 is a schematic diagram showing the timing chart when a WUSB device, operating as an SBD, connects to a WUSB host.
In FIG. 5, the WUSB host 210 sends, after starting up, a beacon from a beacon slot within the BP 400, and reserves a DRP 500. On the other hand, the WUSB device 220, when starting up, performs channel scanning in the superframe N (300, S330), and receives the beacon 700. In addition, the WUSB device 220 analyzes the received beacon 700, and detects a beacon slot that can be used by the WUSB device 220, beacon synchronization timing, etc.
In a superframe N+1 (301), the WUSB host 210 and the WUSB device 220 both send a beacon using a beacon slot during the period of the BP 401, and secures a DRP 510. However, the WUSB device 220 sends a beacon using a usable beacon slot detected in the superframe N (300). As with the BP 401 in FIG. 5, synchronization in the superframe period has been established at the point where a beacon is sent and received by a plurality of apparatuses in the same BP.
In a superframe N+2 (302), the type of a DRP 520 is reserved in private in order to establish a WUSB channel. The WUSB host 210 forms a TG in the DRP 520. The WUSB device 220 sends a connect request, and by the WUSB host replying with a connect acknowledgement in a MMC, a process of establishing a WUSB cluster is begun after a superframe N+3 (303).
Described as follows is the process in which superframe synchronization adjustment is further performed between the WUSB device 220 and the WUSB host 211, in the case where the WUSB device 220, which is an SBD, and the WUSB host 210 have already formed the cluster 200. FIG. 6 is a timing chart for when superframe synchronization adjustment is performed with the WUSB host 211 via the WUSB device 220, in the case where the WUSB host 210 has already formed the WUSB cluster 200 with the WUSB device 220, which is an SBD. In other words, FIG. 6 shows the process of synchronization adjustment with an external apparatus when functioning as an apparatus which sends beacon information for adjusting synchronization timing of communication.
In the superframe N (300), the WUSB host 210 and the WUSB device 220 commonly send beacons from their beacon slots in a BP 404, and reserve a DRP 540. The WUSB device 220 also further performs channel scanning of the superframe N (300). Assume a situation in which, while channel scanning, the WUSB device 220 receives a beacon sent during the BP 480 period by the WUSB host 211.
In this situation, the WUSB host 210 and the WUSB device 220 changes the timing of the superframe so that synchronization with the beacon delivery timing of WUSB host 211 is achieved. That is, when a beacon is received from the WUSB host 211, the WUSB device 220 notifies, via a beacon in a BP 405, the starting point of the BPST to the WUSB host 210. The change in the starting point of the BPST is indicated by a BP switch IE in the beacon. Also, a DRP 550 is reserved as a DRP used by an alien. After a pre-determined period, the BPST of all devices in the WUSB cluster 200 is moved in the superframe N+2 (302), and are made to be the same period as the BP of the WUSB host 211. Through this process, superframe synchronization adjustment is established.
As described with reference to FIG. 5 and FIG. 6, a WUSB device operating as an SBD can dynamically form a cluster with a host or other device, in order to perform superframe synchronization adjustment. However, in practice, a beacon is sent and received even in the situation in which superframe synchronization adjustment is not needed. Consequently, in the case where a mobile device, driven by a battery for apparatuses such as digital cameras and PDAs, is operated as a WUSB device that operates as an SBD, electrical power stored in the battery is used rapidly and the duration of use is shortened.
On the other hand, a WUSB device which operates as an NBD has low power consumption as it does not perform sending and receiving of beacons. However, because it cannot synchronize during a superframe, in the case where there exist alien wireless communication apparatuses, a device operating as an NBD is subject to interference from the wireless communication signal emitted by those apparatuses, and throughput is decreased.